1. Field of the Invention
The present invention relates to a television receiver having the function of displaying a plurality of types of images which differ in the aspect ratio.
2. Description of the Background Art
In digital television broadcasting, not only a television program video having a standard aspect ratio of 3:4 but also a television program video having an aspect ratio of 9:16, for example, a high-definition television program video is broadcast.
JP07-46510, A discloses a television receiver selectively assuming a state where an image reproducing operation is performed on the basis of a high-definition television video signal having an aspect ratio of 9:16 and a state where an image reproducing operation is performed on the basis of an NTSC (National Television System Committee) video signal having an aspect ratio of 3:4.
In such a television receiver, a video signal processing circuit for the high-definition television video signal and a video signal processing circuit for the NTSC video signal are selectively connected to a common image display unit, and an image reproducing video signal based on the high-definition television video signal or an image reproducing video signal based on the NTSC video signal are fed to the image display unit.
When the video signal is fed to the image display unit, a vertical deflection signal and a horizontal deflection signal are fed to a deflecting coil provided in the image display unit. When the image reproducing video signal based on the high-definition television video signal is fed to the image display unit, a vertical deflection signal having a frequency of 60 Hz and a horizontal deflection signal having a frequency of 33.75 kHz are fed to the deflecting coil in correspondence with a field frequency 60 Hz and a line frequency 33.75 kHz which the high-definition television video signal has. On the other hand, when the image reproducing video signal based on the NTSC video signal is fed to the image display unit, a vertical deflection signal having a frequency of 60 Hz and a horizontal deflection signal having a frequency of 15.75 kHz are fed to the deflecting coil in correspondence with a field frequency 60 Hz and a line frequency 15.75 kHz which the NTSC video signal has.
In this case, an image reproducing video signal based on a high-definition television video signal forming a screen having an aspect ratio of 9:16 and an image reproducing video signal based on an NTSC video signal forming a screen having an aspect ratio of 3:4 are selectively fed to the common image display unit. A state where a reproduced image corresponding to the high-definition television video signal is obtained on the image display unit or a state where a reproduced image corresponding to the NTSC video signal is obtained thereon is assumed. In either one of the states, a portion where no image is displayed is formed in the image display unit.
In a television receiver comprising an image display unit 10 having an aspect ratio of 3:4, for example, when a television program video having an aspect ratio of 3:4 is displayed, the television program video is displayed on the whole of a display area of the image display unit 10. When a television program video having an aspect ratio of 9:16 is displayed, however, the television program video is displayed inside both upper and lower side edges of the display area of the image display unit 10, as shown in FIG. 1a. Accordingly, portions where no television program video is displayed respectively appear in the upper and lower side edges of the display area of the image display unit 10. In the portions where no television program video is displayed, black bands (letter boxes) 11 are respectively on-screen displayed.
On the other hand, in a television receiver comprising an image display unit 20 having an aspect ratio of 9:16, when a television program video having an aspect ratio of 9:16 is displayed, the television program video is displayed on the whole of a display area of the image display unit 20. When a television program video having an aspect ratio of 3:4 is displayed, however, the television program video is displayed inside both left and right side edges of the display area of the image display unit 20, as shown in FIG. 1b. Accordingly, portions where no television program video is displayed respectively appear in the left and right side edges of the display area of the image display unit 20. In the portions where no television program video is displayed, black bands (side panels) 21 are respectively on-screen displayed.
Under circumstances where a state where the portion where no image is displayed is thus formed on the image display unit is assumed in displaying the reproduced image by the image display unit, when the accumulation time in the state is relatively increased, there occurs a difference in luminescent capability between a portion where no image is displayed (a first area) and a portion where an image is displayed (a second area) in the image display unit because the degree of reduction in the luminescent capability in the second area is higher than that in the first area.
When screen display is performed over the whole surface of the image display unit under circumstances where the difference in luminescent capability thus occurs, luminance in the second area is lower than luminance in the first area so that there occurs a luminance difference between the first area and the second area. In the boundary between the first area and the second area, a stripe-shaped trace, i.e., burn-in is conspicuous due to the luminance difference.
Disclosed as a technique for effectively reducing the degree of the burn-in is a technique for reciprocating the position of an edge in the horizontal direction or the vertical direction of a screen on which a reproduced image is formed in an image display unit in a relatively small movement range in a predetermined long period. That is, as a method of reducing burn-in in an image display unit by a side panel or a letter box used when programs having different aspect ratios are displayed, the side panel or the letter box is moved by slightly moving a deflection circuit.